If one wishes to observe an object embedded in or lying on a lake bed, a difficulty is that the water around the object may be so cloudy, or become so cloudy after attempting to uncover the object, that normal underwater photographic equipment becomes ineffective. The cloudy water prevents or impairs any visual observation.
Such difficulty may be overcome by providing a path of clear liquid between the object and the means of observation. Conveniently, the path of clear liquid used is a liquid stream at sufficient velocity to be simultaneously the means of excavation, i.e. the means by which the object is uncovered.
In order to provide an observing apparatus which will successfully operate in such a clear liquid stream which is at sufficient velocity for excavation, a problem of design arises. Thus when any body is immersed in a liquid stream, as the liquid passes over the downstream end of the body, the liquid tends not to adhere or conform entirely to the shape of the body. This creates a negative pressure in, or approaches a vacuum at the region downstream of the body which causes vapourization of the liquid in this downstream region, forming bubbles. This bubble formation is referred to as cavitation and a zone of cavitational probability can be defined as the space in which cavitation is likely to occur. The bubbles collapse and disappear a short distance after clearing the zone of cavitational probability, where the negative pressure is no longer present. These bubbles are, of course, between the observing apparatus, such as a camera, and the object and therefore obscure the desired image.
Cavitation becomes a problem for example when a shipwreck, buried in silt or sand, is to be excavated by means of a high velocity water jet (i.e. of sufficient velocity to dislodge loose particulate matter) and it is desired to monitor the excavation whilst the excavation is in progress. At higher liquid velocities, such as are required for excavation purposes, prevention of cavitation is a significant factor. Thus if a video camera for example is positioned within the water jet in order to observe any object falling in the path of the jet, cavitation at the downstream end of the camera will impair observation.
It has now been discovered that in order to prevent or reduce cavitation downstream of an observing apparatus within a liquid stream of sufficient velocity for excavation, it is necessary to modify the shape of the downstream end of the apparatus, so that it substantially assumes the shape of the zone of cavitational probability which would otherwise be formed. Although this modified shape may be a parabolic shape, it is to be understood that the determining factors which define the shape arise from a consideration of the pressure vector components of the liquid stream as it travels past the immersed apparatus. Thus if, for example, observation is effected by means of a video camera and accessories, arranged in a streamlined cylindrical cylinder, with the viewing lens at the downstream end, then by attaching to the downstream end of the container a transparent cover which is substantially the same shape as the zone of cavitational probability of the downstream end, the zone is separated from the liquid stream and cavitation can thus be reduced or eliminated, making it possible to obtain an unimpaired image of an object downstream of the video camera.